Prosthetic devices adapted to be used at a metacarpophalangeal joint (hereinafter MP joint) of a human hand are of two general types, namely, those which act as spacers to facilitate biologic reconstruction of a joint out of fibrous (scar) tissue, and those of more rigid mechanical design which attempt to reconstruct one or several of the axes of motion of the normal MP joint. The former type, illustrated by the Silastic (a registered trademark of DOW Chemical Company for silicone rubber) rubber design of Swanson (hereinafter described), in general, does not place undue stress on the bone and soft tissues into which it is implanted. The latter type, illustrated by the Steffee and Flatt designs (hereinafter described) are constructed of metal-plastic or metal structures, respectively, are in general prosthetic articulations which are constrained at the junctions of their metacarpal and phalangeal parts in such a manner that they transfer torsional and/or bending loads from one part to the other. This results in the parts becoming essentially rigidly coupled together, thereby resulting in mechanical (fatigue) failure of either the prosthesis itself or the biologic tissue, i.e., bone, into which the prosthesis is implanted. In addition, none of the prostheses currently in use substantially reconstruct the biomechanical mechanisms through which the normal MP joint moves and dynamically stabilizes itself.
Prosthetic devices for the metacarpophalangeal joint have been devised by a number of individuals including Flatt, Swanson, Niebauer, Steffee and Walker. A discussion of the Flatt prosthetic device is found in a textbook entitled, "The Care of the Rheumatoid Hand," by Adrian E. Flatt (C.V. Mosby Company, St. Louis 1974) pp 160-166. Flatt's device includes two elongated members for insertion in two adjacent bones of a joint, the members being connected by a single hinge allowing only one degree of freedom of the prosthetic joint. Experience with the Flatt prosthesis has demonstrated either mechanical failure of the prosthesis itself or of the metacarpal and phalangeal bones into which its stems are inserted.
The Swanson device is discussed in a text entitled, "Flexible Implant Resection Arthroplasty in the Hand and Extremities," by Alfred B. Swanson (C.V. Mosby Company, St. Louis 1973) Chapter 8, pp. 147-159. This device comprises a one-piece member of silicone rubber which allows movement of one bone with respect to the other with multiple degrees of freedom. The flexible implant serves to align the skeletal elements while allowing mechanical motion to occur. Biologic encapsulation together with the structural nature of the implant itself serve to reconstruct motion at the level of the resected joint. In the Swanson design, no discrete axis of motion is reconstructed. Shearing forces and bending moments are imparted to the prosthesis itself resulting in eventual (fatigue) failure of a significant number of the implants.
The Neibauer device is similar in construction to that of the Swanson device except that the silicone rubber is integral with a fabric belt in the midportion of this one-piece member prosthesis. The narrow midportion junction of the Neibauer prosthesis forms a hinge which reconstructs the flexion-extension axis of the joint. Unlike the smooth surfaced Swanson design, the Neibauer design has a fabric weave integral with the surface of the stem portions of the same which facilitates a mechanical bond of the prosthesis to the intramedullary cavities of the adjacent bones.
The Steffee design is disclosed by U.S. Pat. No. 3,506,982. This device effectively reconstructs the flexor and extensor moment arms at the level of the MP joint but its mechanical features transfer torsion and bending loads between stem members resulting in either mechanical failure of the prosthesis itself or of its mechanical junction with the adjacent metacarpal and phalangeal bones in a significant number of clinical cases.
The Walker design is disclosed in a text entitled, "Human Joints and Their Replacements", by Peter S. Walker (Charles E. Thomas, Publishers, Springfield, Ill. 1977). The Walker device comprises a plastic metacarpal stem, metal phalangeal stem, and a snap-in plastic axle. The stems have a circular section, with knurling to attach at all points to the cement. The plastic head is part of a sphere, and has a dorsal-volar slot to accommodate the metal post. An elongated hole through the side of the plastic head, and a circular hole in the metal post, accommodate the axle. The back of the collar of the phalangeal component is flat to locate against the bone, while at the front, the surface is part of a sphere, of only slightly larger radius of curvature than the plastic head. When the joint is assembled, load-bearing occurs between the spherical surfaces and not on the axle. At 0.degree. of flexion, radial-ulnar motion is allowed, but progressively with flexion, this lateral motion decreases because the axle is now restrained.
Because of the drawbacks of the prosthetic devices of the type described, a need has continued for an improved prosthesis which allows for movement of one bone adjacent to a joint relative to a second bone adjacent to the joint without transfer by the stems of the prosthesis of excessive torsion and bending loads to the second bone. By minimizing these loads, there will be substantially no breaking of the bond between the stem members of the device and the bones or the device itself. In addition, the need also exists for an improved prosthetic design to reconstruct the axes about which the normal MP joint moves. None of the currently available prostheses reconstructs the longitudinal rotational axis of the finger as determined by the biomechanics of the normal MP joint.